The present invention relates generally to a wide-angle lens system well corrected for chromatic aberrations, which is used on an image pickup device, and more particularly to a wide-angle lens system having a back focus long enough to be used on particular cameras such as video cameras or digital cameras.
A conventional image pickup lens system using a solid-state image sensor or the like is required to have a back focus long enough to locate an optical low-pass filter, an infrared cutoff filter and so on between the image pickup lens and the solid-state image sensor. Thus, the phototaking lens system is generally of the retrofocus type comprising a negative front lens group and a positive rear lens group. However, this type of lens system produces much off-axis aberrations such as distortion and astigmatism because the lens power profile or distribution is asymmetric with respect to an aperture stop. Such image pickup lens systems, for instance, are disclosed in JP-A's 63-81309 and 63-81310. However, these image pickup lens systems are less than satisfactory in terms of correction of various aberrations although their back focus are well ensured.
Regarding chromatic aberrations and especially paraxially longitudinal chromatic aberration, positive chromatic aberration is produced at the aforesaid front lens group located nearer to an object side of the system rather than to an aperture stop while negative chromatic aberration is produced at the aforesaid rear lens group located nearer to an image plane of the system rather than to the aperture stop. Regarding chromatic aberrations of magnification, negative chromatic aberration is produced at the front lens group, and negative chromatic aberration is produced at the rear lens group, too. Thus, the longitudinal chromatic aberration can be corrected at the front and rear lens groups, but the chromatic aberration of magnification cannot. To achieve the target performance, therefore, a conventional retrofocus type phototaking lens system is built up of a front lens group and a rear lens group, each comprising a plurality of positive, and negative lenses, whereby the amount of longitudinal chromatic aberration and chromatic aberration of magnification produced is kept small. Such a phototaking lens system uses a cemented lens for correction of the aberrations, as disclosed in JP-A's 7-27973 and 9-49968 for instance. According to the examples given therein, however, some considerable chromatic aberration of magnification is produced. In particular, the chromatic aberration of magnification with respect to the g-line becomes very large toward the periphery of the field angle.
With the recent progress in production technologies, solid-state image sensors are now increasingly miniaturized to one pixel size of the order of a few .mu.m. With this, phototaking lenses of ever-higher performance are demanded, and it is desired to reduce chromatic aberration of magnification to a very low level in the visible wavelength region. To make good correction for the chromatic aberrations of magnification inclusive of that with respect to the g-line, it is thus required to use anomalous dispersion glasses. Such image pickup lens systems, for instance, are disclosed in JP-A's 5-134174, 7-181376 and 7-248447. In the examples given therein, about two or three anomalous dispersion glasses are used to make correction for chromatic aberrations of magnification inclusive of that with respect to the g-line. However, such anomalous dispersion glasses are expensive, and so phototaking lenses formed of them cost much.
Among recently developed chromatic aberration correcting elements, attention is now paid to a diffractive optical element (DOE for short) harnessing a diffraction phenomenon. Unlike a conventional refractive lens, the DOE is optically characterized by reciprocal dispersion and anomalous dispersion, as expressed by an Abbe's number .nu.=-3.45 and a partial dispersion .theta..sub.gf =0.296. For instance, this is described in an article: Thomas Stone and Nicholas George, "Hybrid diffractive-refractive lenses and achromats", Applied Optics, 27, 14, 2960-2971 (1988.7.15). As well known, this property can be used to cement together a refractive lens of positive power and a DOE of positive weak power, thereby making correction for chromatic aberrations. In recent years, various image pickup lenses have been embodied making use of this DOE property. In the examples of U.S. Pat. No. 5,148,314, for instance, Chungte W. Chen discloses an eyepiece lens, a Petzval type lens, a lens of large aperture, a telephoto lens, etc., and teaches that chromatic aberrations and off-axis aberrations can be corrected by using DOEs in optical systems.
JP-A's 6-331898, 6-331887, 6-347700 and 6-324262, and Publication of the Translation of International Application No. 8-508116, etc. disclose some applications of DOEs to telephoto lenses, objectives, projection lenses, and photographic standard lenses. A typical application of a DOE to a wide-angle lens system is an endoscope objective as disclosed in JP-A 6-194571. This lens-system is a phototaking lens system peculiar to an endoscope objective that produces very considerable distortion although its field angle is wide, and so is unsuitable for the technical field to which the invention belongs.
Never until now is a wide-angle lens system using a DOE to make good correction for monochromatic aberration and chromatic aberrations proposed. Nor is a low-cost yet high-performance wide-angle lens system having a sufficient back focus put forward.